Electrochromic (EC) devices are devices that change light (and heat) transmission properties in response to voltage applied across the device. Metal oxide based electrochromic devices can be fabricated which electrically switch between transparent and absorbing states (where the transmitted light may be colored). Furthermore, certain transition metal hydride electrochromic devices can be fabricated which switch between transparent and reflective states. Electrochromic devices are incorporated in a range of products, including architectural windows, rear-view mirrors, and protective glass for museum display cases.
A prior art electrochromic device 100 is represented in FIGS. 1 & 2, which show a schematic representation of the electrochromic device illustrating ion conduction between anode and cathode, and a cross-sectional representation of the electrochromic device, respectively. See Granqvist, C.-G., Nature Materials, v5, n2, February 2006, p 89-90; C.-G. Granqvist Handbook of Inorganic Electrochromic Materials, Elsevier, 1995; and U.S. Pat. No. 5,995,271 to Zieba et al. The device 100 comprises a glass substrate 110, lower transparent conductive oxide (TCO) layer 120, a cathode 130, a solid electrolyte 140, a counter electrode (anode) 150, upper TCO layer 160, a protective coating 170, a first electrical contact 180 (to the lower TCO layer 120), and a second electrical contact 190 (to the upper TCO layer 160). Furthermore, there may be a diffusion barrier layer (not shown) between the glass substrate 110 and the lower TCO layer 120, to reduce the diffusion of ions from the glass substrate into the TCO layer, and vice versa. Note that the component layers are not drawn to scale in the electrochromic devices shown in FIGS. 1 & 2. For example, a typical glass substrate is of the order of a millimeter thick and a typical electrochromic device covers the fully exposed area of the architectural glass, or rear-view mirror, for example. Other substrate materials may be used, for example plastics such as polyimide (PI), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). Typical component layer thicknesses are given in the table below:
Component LayerThickness (microns)lower TCO layer0.1 to 1.0cathode0.03 to 1.0 solid electrolyte0.005 to 0.5 counter electrode0.03 to 1.0 upper TCO layer0.1 to 1.0diffusion barrier layer0.1 to 1.0
Switching from a transparent to a colored state for the EC device occurs when ions (such as lithium or hydrogen ions) are driven from the counter electrode 150, through the (non electrically conductive) solid electrolyte 140, to the cathode 130. The counter electrode 150 is an ion storage film, and the cathode 130 is electrochromic—providing the desired change in light transmission properties. It is also possible for the counter electrode 150 to function as the electrochromic layer if this layer undergoes an “anodic coloration,” where the layer changes from transparent to colored with de-intercalation of the ion. In this case, the cathode becomes the counter electrode. One can also create greater contrast by combining the effects of both electrodes. A more detailed discussion of the functioning of electrochromic devices is found in Granqvist, C.-G., Nature Materials, v5, n2, February 2006, p 89-90 and C.-G. Granqvist Handbook of Inorganic Electrochromic Materials, Elsevier, 1995. For the device to function properly, the lower TCO layer 120 and the cathode 130 must be electrically isolated from the counter electrode 150 and upper TCO layer 160. Electrical contact to external driver circuits is made through the first and second electrical contacts 180 and 190.
The performance of an EC device relies on the transport properties for ions (H+, Na+ and Li+) and optical behaviors of the electrode and electrolyte materials, and other layers in the device. In order to enhance these properties, the electrode layers are typically doped with various elements from the periodic table, of which the most prominent ones are the transition metals. As the desired layers are typically metal oxides, the “doped” layers are typically deposited with reactive sputtering methods with doped metallic target materials. In addition, there is typically a separate step to intercalate the charge carriers—commonly Li—into the electrodes. The intercalation can be done with either wet or dry lithiation. In either case, the manufacturing process can be severely hampered by the need to deal with reactive Li metal and with procedures to wet lithiate the electrodes.
A typical TFB device structure 300 is shown in FIG. 3, where anode current collector 360 and cathode current collector 320 are formed on a substrate 310, followed by cathode 330, electrolyte 340 and anode 350; although the device may be fabricated with the cathode, electrolyte and anode in reverse order. Furthermore, the cathode current collector (CCC) and anode current collector (ACC) may be deposited separately. For example, the CCC may be deposited before the cathode and the ACC may be deposited after the electrolyte. The device may be covered by an encapsulation layer 370 to protect the environmentally sensitive layers from oxidizing agents. See, for example, N. J. Dudney, Materials Science and Engineering B 1 16, (2005) 245-249. One or more of the electrodes incorporates a mobile ion, such as Li+—for example Li+ in a LiCoO2 cathode. The same concerns discussed above for electrochromic devices may apply to thin film battery fabrication.